(1) Field of the Invention
This invention relates to the fabrication of integrated circuit devices and more particularly to a method of local oxidation using a nitrogen implant to suppress growth of a bird's beak under field oxide in the fabrication of integrated circuits.
(2) Description of the Prior Art
Local oxidation of silicon is the conventional lateral isolation scheme. The conventional local oxidation process (LOCOS) is described in VLSI Technology, International Edition, by S. M. Sze, McGraw-Hill Book Company, NY, N.Y., c. 1988 by McGraw-Hill Book Co., pp. 473-474. Referring to FIG. 1, a layer of silicon nitride 3 is deposited over a pad oxide 2 overlying a silicon substrate 1. The pad oxide is a thin thermal oxide which allows better adhesion between the nitride and silicon and acts as a stress relaxation layer during field oxide formation. The nitride and oxide layers are etched to leave openings, not shown, exposing portions of the silicon substrate where the local oxidation will take place. The field oxide 6 is grown within the openings and the nitride and pad oxide layers are removed. This completes the local oxidation.
Bird's beak encroachment is a disadvantage of the conventional LOCOS method. Bird's beak encroachment 7 is caused by the lateral oxidation of silicon along the pad oxide under the nitride layer during the high temperature and long time required by the oxidation process. The bird's beak encroachment reduces the size of the active area. This is critical as device sizes continue to shrink and packing density increases.
U.S. Pat. No. 4,965,221 to Dennison et al teaches the use of oxide or polysilicon spacers and recessed etching to reduce bird's beak encroachment. U.S. Pat. No. 5,369,051 to Rao et al teaches the use of a polysilicon layer between the pad oxide and the silicon nitride layers as well as the use of silicon nitride spacers. U.S. Pat. No. 5,432,117 to Yamamoto teaches the use of a thick silicon nitride layer with thinner edges to prevent bird's beak formation.
Other methods include the use of oxidation blocking films. U.S. Pat. No. 5,447,885 to Cho et al discloses the use of a silicon oxidation blocking film, then forming an oxynitride film by nitrification of native oxide. U.S. Pat. No. 5,429,982 to Chao teaches growing an oxide layer in a nitrogen ambient and annealing the oxide layer to form an oxynitride layer which reduces the bird's beak. Co-pending U.S. patent application Ser. No. 08/705,451 (TSMC-96-026) to Tsai et al filed on Aug. 29, 1996 teaches an oxynitrided pad oxide layer to suppress bird's beak.